Cooling systems are essential for several stages of hydrocarbon processing. For example, it is typical to cool hydrocarbon gas from wellhead temperatures, which commonly range from about 80° C. to about 150° C., down to about 30° C. to about 60° C., prior to dehydrating the hydrocarbon gas and/or separating condensates therefrom. Additionally, cooling is also needed after compressing the hydrocarbon gas which may occur at several stages during hydrocarbon processing.
Cooling systems that have been employed for offshore hydrocarbon processing include air cooling systems, direct seawater cooling systems, and indirect seawater cooling systems.
Air cooling systems are simple and cost effective. Their widespread deployment in offshore hydrocarbon processing facilities is restricted, however, by deck-space limitations.
Direct seawater cooling systems employ a pump to lift the seawater, filter and then circulate seawater into a heat exchange circuit which communicates with a hydrocarbon process fluid. The heat exchangers employed in the heat exchange circuit must be fabricated from high specification metals or metal alloys which are resistant to corrosion by both the process fluid and seawater. Accordingly, direct seawater cooling systems often involve difficult fabrication with high cost materials. Seawater fouling and mechanical integrity issues in the heat exchangers may be a concern.
Indirect cooling systems interpose a cooling medium heat exchange circuit between the hydrocarbon process fluid heat exchange circuit and a seawater heat exchange circuit. The cooling medium in the cooling medium heat exchange circuit is typically clean (non-fouling) and non-corrosive. In the indirect cooling system, seawater is typically pumped, filtered and circulated through a circuit which is in heat exchange communication with the cooling medium heat exchange circuit, thereby cooling the cooling medium. The cooled cooling medium, in turn, is employed to cool process fluid which is brought into heat exchange communication with the cooling medium heat exchange circuit.
The indirect cooling medium heat exchange circuit represents an additional heat exchange circuit, associated pumps and make-up capability in comparison with the requirements of the direct cooling system. However, the materials used in the heat exchange circuits do not need to meet the high specification requirements for materials used in the direct cooling system, and heat exchange designs can therefore use larger heat exchange areas (and therefore increase the efficiency of the circuit) for comparative costs. Additionally, the risk of fouling is limited to the seawater heat exchanger only. The cooling medium is a very clean fluid and does not contribute to fouling.
Both direct and indirect cooling systems feature a seawater heat exchange circuit in which seawater is filtered and then pumped onto the offshore processing facility platform, circulated in a heat exchange circuit, before being discharged back into the sea. The filters, pumps, and heat exchange circuits have significant capital and operating expense requirements.
Various devices have been proposed for cooling hydrocarbons subsea, in particular in situations where the hydrocarbons are sourced from subsea wells, as distinct from ‘dry tree’ platform wells, when the wells are brought up to either fixed or floating platform facilities. Examples where cooling is required includes occasions where the hydrocarbons are from high pressure/high temperature (HP/HT) wells, when cooling is required prior to the hydrocarbons entering export pipelines, and as part of various subsea processing schemes including subsea separation and subsea compression.
The cooling devices generally send the hydrocarbons down a network of pipes from which cooling results from energy loss through the pipe wall to the surrounding sea. There are several significant disadvantages from current proposals:                It is difficult to control the hydrocarbon temperature, although this may be sometimes achieved by bypassing a fraction of the hydrocarbon. Where cooling is too great and the hydrocarbon fluid flow is low there is a risk of hydrate formation.        The pipe network is invariably made from a high alloy stainless steel or equivalent which can be prohibitively expensive.        Apart from hydrates, the pipe cooling network can be pre-disposed to build up of wax and sometimes sand in periods of reduced flowrates and operating temperatures. These potential blockages are difficult to remove from the pipe network configuration.        
One example of such device includes employing a subsea cooling loop to circulate a process fluid through a length of pipe located subsea to directly cool the process fluid and then pump the cooled process fluid back to the offshore processing facility platform for processing and export.
The present invention seeks to overcome at least some of the aforementioned disadvantages.